A new article from SEMRush named How to Add FAQ Schema to Any Page Using Google Tag Manager goes through the steps on how to add FAQ schema to your pages using Google Tag Manager. When Cindy Krum asked Google's John Mueller about this on Twitter, John said "Reading the linked article, it seems quite complete and balanced - I think it's fine." He even added "Happy to see more sites trying out new structured data types!"

Here are those tweets:

Reading the linked article, it seems quite complete and balanced - I think it's fine. It also touches on the aspect you mentioned, using GTM vs on-page markup. Happy to see more sites trying out new structured data types!